1. Field of the Invention
The invention is directed to optical fiber. More specifically the invention is directed to highly nonlinear optical fiber (HNLF) and ways to reduce the threshold of stimulated Brillouin scattering (SBS) in HNLF.
2. Description of Related Art
Highly nonlinear optical fibers (HNLF), defined as fibers with an increased nonlinear coefficient (γ) and an engineered dispersion close to zero, have been used in numerous applications, including parametric amplification, phase sensitive amplifiers, optical regenerators, switching, and wavelength conversion. One of the limitations of conventional HNLF is stimulated Brillouin scattering (SBS). SBS is a phenomenon where, when light having power over a certain threshold is launched into an optical fiber, a part of the light is reflected and, therefore, the light cannot propagate through the fiber at high intensity. As a result, the intensity of the transmitted light is restricted (and the efficiency of the optical device is restricted), which sets an upper limit on the amount of power which can be launched into the HNLF. The SBS threshold power (Pth) determines the maximum obtainable nonlinear phase shift, which is given as:γLeffPth  (1)where Pth is the threshold power for the onset of SBS, γ is the nonlinear coefficient, and Leff the effective length, given by:
                    γ        =                              2            ⁢            π            ⁢                                                  ⁢                          n              2                                            λ            ⁢                                                  ⁢                          A              eff                                                          (        2        )                                          L          eff                =                              1            -                          exp              ⁡                              (                                                      -                    α                                    ⁢                                                                          ⁢                  L                                )                                              α                                    (        3        )            where n2 is the nonlinear refractive index, λ is the wavelength, Aeff the effective area, α is the loss coefficient in units of [m−1], and L is the fiber length. Expression (1) is often referred to as the “figure of merit” for SBS-limited HNLFs. For conventional HNLFs having a germanium-doped silica core, the figure of merit is approximately 0.21, which is too low for some applications, such as phases sensitive amplifiers, and compression of the beat signal from two lasers closely spaced in wavelength and operating in continuous wave mode. It would be desirable to raise the SBS threshold for HNLFs.
Another way to increase the figure of merit PthγLeff is to increase the nonlinear coefficient γ by making HNLFs from soft glass materials such as bismuth or lead silicate rather than silica. Quite high figure of merits, that is, FOMs greater than 1 have been demonstrated with soft glass based HNLFs. A drawback of such fibers is the quite high losses (approximately 1 dB/m) and the high coupling loss as compared to standard single mode fibers.
Two somewhat similar techniques for increasing the SBS threshold include applying a linear or stepwise temperature gradient, or a linear or stepwise strain gradient along the fiber. The principle in both techniques is that temperature or strain changes the SBS frequency shift. By applying a temperature/strain gradient along the fiber, the Brillouin line width is broadened and as a consequence the Brillouin gain coefficient gB is lowered. Increases in threshold of approximately 8 dB have been demonstrated in a HNLF using the temperature technique. Using the straining technique on HNLFs, threshold increases of 6-7 dB have been demonstrated. A drawback in both techniques is that both strain and temperature may also alter the dispersion. Both techniques have drawbacks from a practical point of view. Large strain will reduce the fiber lifetime. Unless very large diameter spools are used, straining will also increase the polarization mode dispersion (PMD) of the HNLF.
Another method to reduce the SBS in HNLF is to dope the core of the HNLF with aluminum instead of germanium. However, this method does not address the issue of attenuation, and a fiber having an effective area of 15 μm2 or smaller requires additional undesired power requirements resulting from the high attenuation. Such methods tend to focus exclusively on the wt % doping level of aluminum in the core and fluorine in the trench region. However, there is no agreement in scientific literature as to the relationship between how much aluminum or fluorine is used and the respective changes that doping creates to the refractive indices of the thus-doped components. As an example, although there is theoretically a linear relationship between wt % amount of doped material and change to the refractive index of silica, literature values for the fluorine proportionality coefficient range from −4×10−3/% to −8×10−3/%, and values for the aluminum proportionality coefficient range from 1.731×10−3/% to 2.76×10−3/%. There are several possible reasons for such a large variation in the reported proportionality coefficient, including measurement uncertainty, dependence of the thermal history of the glass, and dependence of the amount of stress in the glass, among other possible reasons. Thus, one cannot properly rely on wt % levels estimated from index profile measurements.
Additional attempts to dope the core of an HNLF with aluminum instead of germanium thus far have resulted in quite high attenuation, which will limit the usable length and consequently increase the requisite pump power.
Accordingly, there is a long felt need to provide a HNLF that exhibits a higher SBS threshold than previously exhibited without evidencing attenuation problems.